Of the 31 states that have declared their opposition to taking in Syrian refugees, one state, Kentucky, has a specific reason to be wary of the background check process: previously two Iraqi refugees who settled in Bowling Green turned out to be al Qaeda-linked terrorists with the blood of American soldiers on their hands, an ABC News investigation found. Both pleaded guilty to terror-connected charges after trying to acquire heavy weapons while in America’s heartland.

The 2013 ABC News investigation also revealed that several dozen other suspected terrorist bombmakers, including some who were believed to have targeted U.S. troops, may have mistakenly been allowed to move to the U.S. as Iraq and Afghanistan War refugees, among the tens of thousands of innocent immigrants.

The Obama administration insists now that Syrian refugees are subjected to intense vetting before they’re allowed to settle in the U.S. and that a vast majority of the millions of refugees the U.S. has resettled since the 1970s are normal, peaceful people, but the program has had serious security problems before. In 2009, a flaw in background screening of Iraqi refugees allowed the two al Qaeda-linked terrorists to settle in Bowling Green and led to a temporary suspension of the refugee program, officials told ABC News in a 2013 investigation.

The two men, Waad Ramadan Alwan and Mohanad Shareef Hammadi, were caught on surveillance video in 2010 in a storage locker in Kentucky handling heavy weapons, including a Russian-made machine gun and a Stinger missile launcher, which the FBI said the men thought would be smuggled to insurgents in Iraq.

An FBI agent assigned to the sting operation that eventually nabbed Alwan and Hammadi told ABC News for its original report that Alwan had bragged to an informant about killing American soldiers in Iraq.

"He said he had them 'for lunch and dinner,'" FBI Louisville Supervisory Special Agent Tim Beam told ABC News in 2013.

Some officials have claimed publicly that the suspects intended to attack a U.S. military post in Kentucky, but FBI officials told ABC News the men only discussed using a bomb to kill a U.S. Army captain they had known in Iraq but did not take any action toward killing him.

Alwan had been arrested in Kirkuk, Iraq in 2006 and confessed during questioning that he was an insurgent, according to the U.S. military and the FBI, which obtained a tape of the interrogation during the government’s 2011 investigation. But he had been released by Kurdish authorities, the tape ended up in a U.S. military database that was never accessed by refugees screening officials, and in 2009 Alwan applied as a refugee to the U.S. He was allowed to move to Bowling Green, home to a refugee program that’s successfully resettled thousands of desperate, peaceful people.

The FBI caught back up to Alwan after receiving a intelligence tip about him and concocted a sting operation in which Alwan would be led to believe the weapons he was obtaining would be shipped to al Qaeda-Iraq, which eventually renamed itself the "Islamic State," or ISIS. The FBI strung Alwan and his accomplice, Hammadi, along, with then-FBI Director Robert Mueller personally briefing President Obama on the case, until investigators were able to determine there were no other co-conspirators, at which point the two were arrested. Both Iraqis pleaded guilty to terror-connected charges and received multi-decade sentences.

"We need to take this as a case study and draw the right lessons from it, and not just high-five over this," said retired Army Lt. Gen. Michael Barbero, who headed the military's Joint IED Defeat Organization until May 2012. "How did a person who we detained in Iraq -- linked to an IED attack, we had his fingerprints in our government system -- how did he walk into America in 2009?"

In the wake of the Kentucky case, the U.S. halted the refugee program for Iraqis for six months, a fact the Obama administration did not disclose to Congress at the time, officials told ABC News in the 2013 investigation.

The U.S. takes in as many as 70,000 refugees from around the world every year – a vast majority of whom are never deemed national security threats -- and the White House recently announced it plans to increase that to 100,000 per year by 2017.

The security concerns over Syrian refugees -- millions of desperate people fleeing widespread violence in their homeland -- boiled over this weekend after it was revealed that European authorities believe one of the suicide bombers who helped kill more than 120 people in Paris last week had used a Syrian passport with likely a fake name to slip into Europe hidden among hundreds of innocents.

The governors of more than half the states in the U.S. now have objected to accepting Syrians fleeing one of the worst humanitarian crises in modern history, and lawmakers such as House Homeland Security Committee Chairman Michael McCaul, R-Texas, have demanded that President Obama temporarily suspend the program.

At the time of ABC News’ original investigation in 2013, then-Department of Homeland Security spokesperson Peter Boogaard said the U.S. government “continually improves and expands its procedures for vetting immigrants, refugees and visa applicants, and today [the] vetting process considers a far broader range of information than it did in years past.”

Tuesday senior Obama administrations official defended the U.S. refugee program, saying those coming into the states are subjected to “the most rigorous screening and security vetting of any category of people entering the United States."

Tweeting today, President Obama said, “We will provide refuge to at least 10,000 refugees fleeing violence in Syria over the next year after they pass the highest security checks. Here, our focus is giving safe haven to the most vulnerable Syrians – women, children, and survivors of torture. Slamming the door in the face of refugees would betray our deepest values. That’s not who we are. And it’s not what we’re going to do.”